Various types of spinal column disorders are known and include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal backward curvature of the spine), excess lordosis (abnormal forward curvature of the spine), spondylolisthesis (forward displacement of a lumbar vertebra) and other disorders, such as ruptured or slipped discs, broken or fractured vertebrae and the like. Patients who suffer from such conditions usually experience extreme and debilitating pain. A technique known as spinal fixation uses surgical implants which mechanically immobilize areas of the spine assisting the eventual fusion of the treated vertebrae. Such techniques have been used effectively to treat the above-described conditions and, in most cases, successfully elevate the pain suffered by the patient.
One technique for spinal fixation includes the immobilization of the spine by the use of a pair of spine rods that run generally parallel to the spine. In practicing this technique, bone screws are first fastened to the pedicles of the appropriate vertebrae or to the sacrum and act as anchor points for the spine rods. The bone screws are generally placed two per vertebra, one at each pedicle on either side of the spinous process. Clamp assemblies join the spine rods to the screws. The spine rods are generally custom-bent to achieve the desired curvature of the spinal column. Examples of such spinal fixation devices can be found in U.S. Pat. Nos. 4,653,481 and 5,030,220, which are hereby fully incorporated herein by reference. These types of systems are very stable but require implanting screws into each vertebra over the area to be treated. Also, since the pedicles of vertebrae above L2 are relatively small, only small screws can be used. To stabilize the implanted system sufficiently, one vertebra above and one vertebra below the area to be treated are often used for implanting pedicle screws. Proper insertions of screws into the pedicles is time consuming and tedious. If a screw is not inserted centrally in the pedicle and parallel to the axis of the pedicle, an unstable connection may result. Further, if the screw is directed toward the caudal (bottom) side of a vertebra, the screw may impinge on a spinal nerve.
Other fixation systems use hooks which fasten a spine rod to a vertebra for tension (distraction) or compression. The hook systems which are currently in use attach to the vertebral lamina located between the spinous process and the transverse process (such as that described in U.S. Pat. No. 5,005,562) or to the transverse process (such as that described in U.S. Pat. No. 4,269,178) of the vertebra. These types of locations for attachment of the hooks place load onto parts of the vertebra which lead to the possibility of damage to the vertebra and failure of the system during use.
It is desirable that a fixation device be provided which not only reduces the need to implant pedicle screws into the vertebra but also which connects to the strong sections of the vertebra thus reducing the damage to the vertebra after installation of the fixation system. Preferably, the point of attachment of such a fixation device is to a central portion of the lamina or the intersection of the lamina and the spinous process of the vertebra in the area of the spine to be treated. This area has proven to be the strongest bone structure of the vertebra. It is further desirable to provide various configurations of fixation systems to accommodate the various shapes and sizes of vertebrae, and to provide a method and tools for implanting fixation systems. Such improvements translate directly into shorter surgeries and reduced surgical costs.